Digital cameras have reached practical levels in terms of high number of pixels (high image quality), compactness, and slimness and replaced 35 mm film cameras from the view points of the function and market. As one aspect of further evolution, a further increase in the number of pixels is strongly desired to be achieved along with an increase in the zoom ratio and an increase in the angle of view while keeping the smallness and slimness.
Image forming optical systems that have been used for their advantage in slimming of the optical system include, for example, optical systems having a reflecting optical element that bends the optical path provided in the first lens group counted from the object side. The use of such an image forming optical system allows to make the depth or thickness of the camera body very small (patent documents 1 and 2).
In the optical systems disclosed in patent documents 1 and 2, a reflecting optical element in the first lens group is adapted to surely bend the path of beams having a certain angle of view. In this case, in order to surely provide a reflecting surface having an area needed for the angle of view, the equivalent air thickness of the first lens group G1 along the optical axis is necessitated to be large. Especially when the angle of view is increased, an increase in the equivalent air thickness of this part becomes notable. On the other hand, the larger the aforementioned equivalent air thickness is, the larger the area of the reflecting surface is needed to be. Therefore, in such optical systems, a negative refracting power is provided immediately before the reflecting surface and a positive refracting power is provided immediately after the reflecting surface to decrease the area of the reflecting surface and also decrease the equivalent air thickness to some extent.
Examples of optical systems having a high magnification are disclosed in Japanese Patent Application Laid-Open No. 2006-71993 and Japanese Patent Application Laid-Open No. 2006-209100.
For image forming optical systems in which a high zoom ratio and a wide angle of view can be achieved at the same time, a configuration in which the lens group closest to the object side (i.e. the first lens group) has a positive refracting power, and a lens group that moves along the optical axis during zooming is suitable (patent document 5). However, this lens configuration has a disadvantage that the diameter of the first lens group tends to be large and providing an adequate edge thickness necessitates a large lens thickness along the optical axis. To reduce the diameter of the first lens group, the position of the entrance pupil at the wide angle end may be made closer to the object side.
One method to achieve this is to configure the first lens group in such a way that the position of the principal point of the first lens group is located as close to the image side as possible. Specifically, in the first lens group, a lens component having a negative refracting power is arranged on the object side and a lens component having a positive refracting power is arranged on the image side. In addition, it is preferred that lens components be arranged with large distances therebetween to an acceptable extent and have strong powers (patent document 6). This allows to achieve a wide angle of view. However, telephoto lenses and zoom lenses set to a telephoto zoom position having the above-described configuration suffer from a problem that even when spherical aberration is excellently corrected with respect to the d-line, large negative spherical aberration occurs with respect to the g-line and the h-line.
Image forming optical systems that have been used heretofore for their advantage in sliming of the optical system include, for example, a system in which a reflecting optical element that bends the optical path is provided in the lens group closest to the object side (or the first lens group) (patent documents 1 and 2). However, increasing the zoom ratio while maintaining the slimness will make the entrance pupil farer from the object side. In addition, increasing the angle of view will make it difficult to ensure bending of the beams needed for imaging at all the angle of views. Therefore, it is necessary to make the entrance pupil closer to the object side.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-302576    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-264343    Patent Document 3: Japanese Patent Application Laid-Open No. 2006-71993    Patent Document 4: Japanese Patent Application Laid-Open No. 2006-209100    Patent Document 5: Japanese Patent Application Laid-Open No. 2003-255228    Patent Document 6: Japanese Patent Application Laid-Open No. 11-133303